Added directory to be deleted into maven-clean-plugin in pom.xml.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #2613 from tsudukim/feature/SPARK-3757 and squashes the following commits:

8804bfc [Masayoshi TSUZUKI] Modified indent.
67c7171 [Masayoshi TSUZUKI] [SPARK-3757] mvn clean doesn't delete some files